The invention relates to a method of establishing a subscriber connection and to a subscriber network. In the method a subscriber connection is established by forming a first portion of a transmission connection on the subscriber side in an electrical form and a second portion of the connection on the exchange side in an optical form by means of an optical fibre, whereby there are optoelectric converters at both ends of said second portion of the connection. A subscriber network according to the invention is useful for practicing the method.
Optical fibre is a self-evident choice for a transmission medium of a trunk network, because trunk connections generally require a large transmission capacity, transmission distances used are long and cable routes ready for use are often available. On the other hand, the situation in a subscriber connection is often quite the opposite and sufficient reasons for using optical fibre rarely exist.
Naturally, with a continuously increasing need for speed, the situation is changing in a more advantageous direction for the optical fibre, but no significant savings can be expected in the total costs, mainly resulting from cable installation costs. A wish is, however, to fit out the subscriber network as well with as much optical fibre as possible, because it is obvious that it will be required in the future. Costs of reconstructing a subscriber network are very high and, as to the time, it is a matter of decades in this connection. Consequently, it would be worth while being prepared for coming needs and providing facilities for the future at present already.
Accordingly, teleoperators try to increase the number of fibres in a subscriber network, as far as it is economically possible. In practice there are two possibilities of increasing the number of fibres. Firstly, optical fibre can be installed at relatively low additional costs in connection with cable supplements. However, the fibre often remains awaiting future users. The other way is to find customers who need high speed and thus optical fibre and who are ready to pay for it.
The fact is that high costs are the worst obstacle to introducing the fibre into the subscriber network.
In a so-called active subscriber network, there are transmission devices also "in the field" (in the area between a terminal exchange and a subscriber). The object of an active subscriber network is to divide the costs of constructing a transmission connection among several subscribers and to utilize existing copper cabling as far as possible. FIG. 1 shows the principle of an active subscriber network. A transmission equipment, joining an optical portion and a copper portion of a subscriber connection and having a box indicated by reference mark B, is positioned in the field in such a place where subscribers 12 are reached by means of an existing copper cabling 13. The transmission equipment typically comprises a multiplexer 11 and baseband modems 14 for each subscriber connection. Correspondingly, each subscriber has a modem 14 of its own, which modems can operate at different rates. A connection from the multiplexer 11 to an exchange 16 is formed by means of an optical fibre 15. The maximum length of the copper portion 13 is typically 0.5 . . . 1 km and the length of the optical portion 15 typically 1 . . . 5 km.
By means of the arrangement described above, the costs of establishing an optical fibre cable connection can be divided among several subscribers 12. The costs are further lowered by possible finished pipings, occurring most probably just in the vicinity of exchanges. Consequently, the most expensive part of the connections, i.e. individual connections to subscribers, would consist of existing copper cable connections, which means that they would be practically almost free of cost.
On the basis of crosstalk between the pairs of a pair cable and an attenuation of a copper cable, it can be estimated that, by using one quad, it is possible in practice to achieve a distance of one kilometer at the rate of 8 Mbit/s and a distance of 2,5 km at the rate of 2 Mbit/s. By using 2 or 3 quads and by accepting a distance of about 500 meters, the rate can be even 34 Mbit/s.
By means of the active subscriber network described above, connections faster than 2 Mbit/s are thus achieved at reasonable costs. If an optical cable is terminated at a distance of about one kilometer from the subscribers at the maximum, a conventional G.703/2 Mbit/s interface and a transmission according to that can be used as such (no separate modem is necessary).
However, an active subscriber network has drawbacks, too, which will be described in the following. Firstly, the large size of the devices leads to the fact that it may be difficult to find a place in the field where the devices required can be positioned. Additionally, the structure of the devices ought to be modular so that interfaces of different kinds can be provided. Simultaneously, the structure ought to withstand hard environmental conditions. Moreover, the devices consume so much power that it is not possible in practice to feed such a quantity of power all the way from the exchange.
Control over the devices constitutes a further problem. There is one more place in a transmission connection of an active subscriber network into which a modem required shall be introduced. It is necessary to know how to configurate this modem right or how to arrange a possibility of remote configuration. This concerns multiplexers, too. Additionally, it shall be possible to control the operation of the devices, and e.g. changes in configuration for one subscriber must not cause any harm to the other subscribers. It is also very laborious to maintain and replace transmission devices in the field.